1. Field of the Invention
The present invention relates to an image recording apparatus in a facsimile communicating apparatus or the like and, more particularly, to an image recording apparatus using thermal recording means.
2. Description of the Prior Art
Hitherto, in such apparatuses, a temperature detecting device such as a thermo-couple, a thermistor, or the like is attached at a proper position of a thermal head in order to keep a temperature of a heating element constant, and energy which is applied to the heating element is changed on the basis of the temperature data measured. However, the temperature of the heating element is not constant due to various factors and a density difference occurs in the reproduced image depending on the mode thereof. One of those factors which is particularly significant is based on the point that the recording period of a line varies depending on the mode of the image to be reproduced.
For example, when considering the cases of the received image and the reproduced image resulting from copying operation, in the case of the received image, in general, it takes some time to decode the MH (Modified Huffman) code or MR (Modified Read) code or to convert the RL (Run. Length) code to original code in accordance with the equipment, or the like. Further, in the case of the received image, because of the limitations of transmission rate and transmission time itself, as the pattern of the image is complicated, the recording time of each line becomes so long as to exceed the capability of the shortest recording time which the recording apparatus has and it takes a long time until the heating element gets cool, so that its temperature does not decrease.
On the other hand, in the case of the copying operation, there is no need for decoding or to convert the data; therefore, the recording apparatus performs the recording in the shortest recording time. This results in an increase of the temperature of the heating element due to a heat regenerative effect thereof and there is a drawback such that the reproduced image becomes thicker than the reproduced image upon reception. Due to this, if an energy is applied to the heating element preliminarily in accordance with the copying operation, the reproduced image upon reception will have become thinner than expected. At this time, it is preferable to instantaneously detect the temperature of the heating element and control the energy to be applied on the basis of the temperature data detected. However, it is difficult to make such an ideal temperature sensor in terms of the structure of the thermal head.
On one hand, in facsimile apparatuses, there has been known the system in which a density of scanning lines is switched in accordance with the object and thereby to efficiently transmit image information. For example, in apparatuses based on the international standards, there have been known the scanning line densities of two stages of 7.7 lines/mm called a fine mode, and 3.85 lines/mm called a standard mode. The switching of the scanning line density is performed by changing the scanning density due to the movement of a recording paper or a recording head. Therefore, if the same amount of electric power is applied to the recording head in the fine and standard modes, the density of characters or image recorded will obviously become thick in the fine mode and thin in the standard mode.
In addition, facsimile apparatuses have a feature that the period when the image data of one line is derived (hereinafter, this period is referred to as a recording period) differs in dependence on the reading speed of the apparatus on the distant side, transmission mode such as G2, G3, or the like which is used, various data processing times for a half tone image process and the like, compression processing time, and the like, respectively. Therefore, the heat regenerative amount of the recording head varies in dependence on the recording period, so that there occurs a problem that the constant recording density is not obtained, depending on the transmission mode, image processing method, or the like.
The foregoing problems are the common problems in thermal recording apparatuses which are used in equipment having a possibility that the recording density or recording period of the input data varies as well as the facsimile apparatuses.